Stromatolites and calcareous structures found in the slate-limestone series (Precambrian? or lower Paleozoic?) of the Niari area in the central Congo region, French Equatorial Africa, are without chronological value, inasmuch as they are facies fossils. Nevertheless they are often valuable stratigraphically; several beds in which they are abundant can be used as horizon markers, particularly locally. The origin and classification of stromatolites are reviewed briefly.